Projection screen fabrication apparatus and method

ABSTRACT

A method and apparatus for fabricating projection screen masters from which highly efficient projection screens of improved aesthetic quality can be replicated. The cutting stylus of a sound-recording head is used to cut a plurality of contiguous grooves in the surface of a blank master. A signal of predefined waveform but of random frequency is applied to to the cutting stylus to modulate the cutting depth thereof in such a manner as to produce image light-redistribution microelements of various sizes but of substantially identical optical power.

United States Patent 1 Hoadley et al.

[ Jan. 29, 1974 PROJECTION SCREEN FABRICATION APPARATUS AND METHOD [75]Inventors: Harvey O. Hoadley; Robert N.

Wolfe, both of Rochester; Beverly F. Palmer, Webster; Roger S.Vanheyningen, Rochester, all of NY.

[73] Assignee: Eastman Kodak Company,

Rochester, NY.

22 Filed: Dec. 13, 1971 21 Appl. No.: 207,383

[52] US. Cl. 83/5, 83/701, 90/13 C [51] Int. Cl B26d 3/06 [58] Field ofSearch 83/5, 701; 51/59 SS, 59 R;

82/D1G. 9; 90/13 C [56] References Cited UNITED STATES PATENTS 2,404,2227/1946 Doner 83/5 8/1960 Sculley 83/5 X 10/1945 Scholz 90/13 C PrimaryExaminer-J. M. Meister 5 7] ABSTRACT A method and apparatus forfabricating projection screen masters from which highly efficientprojection screens of improved aesthetic quality can be replicated. Thecutting stylus of a sound-recording head is used to cut a plurality ofcontiguous grooves in the surface of a blank master. A signal ofpredefined waveform but of random frequency is applied to to the cuttingstylus to modulate the cutting depth thereof in I such a manner as toproduce image light-redistribution microelements of various sizes but ofsubstantially identical optical power.

7 Claims, 10 Drawing Figures PATENTEB'JAN 2 9 I974 SHEET 1 OF 6 HARVEY0. HOADLE) ROBERT/V WOLFE BEVERLY EPALMER ROGER S. l cmHEYN/NGENINVENTORS PAIENIEU 3.788.171

sum 2 or 6 ROBERT/V. WOLFE BEVERLY E PALMER ROGER 5. Van HE Y/Vl/VGE/VINVENTORS WWW ATTORNEY PROJECTION SCREEN FABRICATION APPARATUS ANDMETHOD CROSS REFERENCE TO RELATED APPLICATIONS Reference is made to thecommonly assigned copending applications, Ser. No. 207,082, filedconcurrently herewith in the names of J. .l. DePalma et al, entitledRadiation-Redistributive Devices, and Ser. No. 207,084, filedconcurrently herewith in the names of H. O. Hoadley et al, entitledProjection Screen.

BACKGROUND OF THE INVENTION The present invention relates to thefabrication of front and rear projection screens of the type comprisinga multitude of contiguous optical microelements, each being speciallycontoured to distribute image flux in such a manner that its luminancewill be substantially constant wherever viewed within a predefinedcommon solid audience angle. More particularly, the present inventionrelates to the fabrication of screen masters from which aestheticallypleasing screen surfaces of high efficiency can be replicated.

In the commonly assigned copending application Ser. No. 207,082, filedconcurrently herewith in the names of J. J. DePalma et al, there isdisclosed a projection screen having an image-light-distributing surfacewhich comprises a plurality of contiguous grooves. The depth of each ofsuch grooves undulates in a periodic manner material the groove lengthin accordance with a predetermined waveform to define a row of opticalmicroelements which, in a preferred embodiment, alternate from concaveto convex in shape. Each microelement is substantially identical insize, whether concave or convex, each having a contour such as toredistribute incident image requisite such that its luminance issubstantially constant everywhere within predefined horizontal andvertical audience angles. Also disclosed in the same application is amethod for fabricating such a screen surface. Briefly, the methodcomprises cutting the grooves in the surface of a blank master with thestylus of a sound-recording head, while electronically modulating thecutting position of the stylus with an electrical signal of suchwaveform as to produce the groove depth profile desired when applied'tothe recording head.

While the projection screen described in the above application performsquite satisfactorily in distributing image light, it has been found thatthe appearance of such a screen is not, when illuminated with imagelight or room lighting, aesthetically pleasing when manufactured inaccordance with the method disclosed. Ideally, the depth profile of eachgroove should be perfectly in phase with that of all other grooves.However, since the grooves are necessarily cut in a sequential manner,the workpiece from which the projection screen is eventually fabricatedbeing moved past the cutting stylus in a series of equally spacedparallel traverses, it is exceptionally difficult, due to minutevariations in the velocity of the work table and in the frequency of thesignal used to modulate the depth of cut of the cutting stylus tomaintain the ideal phase relationship from one groove to another.Commonly, the screen surface exhibits random streaks of light and darkareas running parallel to the grooves which are unpleasing to the eye.Also, the array of small uniformly-sized microelements may give rise todiffraction fringes which, in turn, may

cause a microelement to appear lighter or darker than it should,depending on'the specific point in the audience space from which it isviewed, or even introduce color where there should be none.

SUMMARY OF THE INVENTION Accordingly, it is an object of the presentinvention to provide a method and apparatus for fabricating highlyefficient and directional projection screens of improved aestheticqualities.

Another object of the invention is to improve the aesthetic appearanceof projection screen surfaces of the type which comprise a multitude ofspecially contoured optical microelements by providing a method andapparatus for randomly varying the size of the microelements over thescreen surface in such a manner as to give the surface a velvet-likeappearance when viewed under normal room light or projected image light.

Another object of the invention is to improve the uniformity of theluminance and color, when viewed from any point within a predefinedsolid audience angle, of projection screen surfaces of the typecomprising a multitude of optical microelements, by providing a methodand apparatus for randomly varying the size of the microelements overthe screen surface in such a manner as to prevent the formation ofregular diffraction fringes in the light redistributed by such surface.

In accordance with the present invention, the above objects are achievedby employing the cutting stylus of a sound-recording head as a tool forcutting the screen surface and by modulating the cutting position of thestylus in such a manner that, as contiguous grooves are cut in thesurface of a blank masterfthe depth of each groove is caused to undulateat a random spatial frequency and thereby define alternately convex andconcave optical microelements of random length but of substantiallyidentical contour and optical power. Circuitry is provided forfrequency-modulating a predefined stylus-driving signal withlow-frequency noise, and for varying the depth ofcut in accordance withthe instantaneous frequency of the stylus-driving signal.

Other objects of the invention and its various advantages will becomeimmediately apparent to those skilled in the art from the ensuingdetailed description of preferred embodiments, reference being made tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photograph comparing aprojection screen surface fabricated in accordance with the presentinvention with a screen surface fabricated in accordance with'the methodand apparatus disclosed in the aforereferenced application;

FIG. 2 is a side elevational view illustrating a portion of asound-recording head used to fabricate screen masters in accordance withthe present invention;

FIG. 3 is a constructional front elevational view of a portion of astero sound-recording head;

FIG. 4 is a side view of the cuttingstylus of the recording head of FIG.3, illustrating the stylus support;

FIG. 5 is a perspective view of apparatus adapted to translate a blankprojection screen master relative to the screen-cutting apparatusdepicted in FIG. 2;

FIG. 6 illustrates the manner in which the waveform of thestylus-driving signal differs from the stylus motion produced thereby;

FIG. 7 is a block diagram of preferred circuitry for driving the cuttingstylus of a sound-recording head to produce a projection screen masterin accordance with the invention;

FIG. 8 is an electrical schematic of the phase-lagging circuit of FIG.7;

FIG. 9 is an electrical schematic of the amplitude compensation circuitof FIG. 7', and

FIG. 10 is an electrical schematic of the shaping circuit illustrated inFIG. 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS A The photograph of FIG. 1compares the room-light appearance of a projection screen surface 10fabricated in accordance with the method and apparatus disclosed in theabove-referenced application of Hoadley et al, with a projection screensurface 11 fabricated in accordance with the present invention. Surface10 defines a plurality of contiguous grooves of substantially uniformwidth which, as viewed in FIG. 1, run in a vertical direction. The depthof each groove undulates periodically along the groove length at asubstan' tially constant spatial frequency, thereby defining a row ofoptical microelements of substantially equal half wave length,alternating from concave to convex in shape along the groove length.Each microelement, whether convex or concave, is contoured such as toredirect normally incident image flux in such a manner that itsluminance is substantially constant throughout predefined horizontal andvertical audience angles. Because of the uniform groove width andconstant wavelength of depth undulation, each microelement issubstantially the same size as all other microelements. The physicaldimensions of each microelement are such as to be unresolvable by theclosest intended viewer. Typically, microelement width and length varyfrom 0.001 to 0.0 l inch, and the peak-to-peak groove depth isapproximately 0.003 inch.

As indicated in the above-referenced applications, the transverse andlongitudinal cross sections of a convex microelement which is contouredto redistribute normally incident image flux in such a manner as toproduce substantially constant luminance throughout a predefinedaudience space bounded by viewing angles A and B (measured from thenormal in the plane of the cross section) and substantially zeroluminance elsewhere, must be substantially defined by at least a segmentof the curve where n is the refractive index of the microelement (nbeing l in the case where the microelement is reflective); u and w arethe microelement coordinates, w being measured in a direction parallelto the path ofincident radiation, and u being measured in the plane ofthe cross section, perpendicular to w; and w has a value within thefollowing limits:

cos A w 5 I, when f(w;n) is positive and the microelement is refractive,and when f(w;n) is negative and the microelement is reflective; and cosB S w 5 1, when f(w;n) is negative and the microelement is refractive,and when f(w;n) is positive and the microelement is reflective.

Similarly, the transverse and longitudinal cross sections of a concavemicroelement must be substantially defined by at least a segment of thecurve wherein w has a value within the following limits:

I S w 5 cos B, when g(w;n) is positive and the microelement isrefractive, and when g(w;n) is negative and the microelement isreflective; and

l s w s-cos A, when g(w;n) is negative and the micro-element isrefractive, and when g(w;n) is positive and the microelement isreflective.

The depth profile of each of thegrooves comprising surface 10 issubstantially defined by equations (1) and (2) above, and the transversecross section of each of the grooves is substantially defined byequation (2). While the spatial frequency of groove depth undulation issubstantially constant over the screen surface, the phase of suchundulation varies from groove to groove. And while such phase variationhas been found to have substantially no adverse effect on screenperformance, it has been found to give the screen surface a somewhatstreaky appearance which is rather unpleasant aesthetically.

Projection screen surface 11, while having substantially the same lightdistributing qualities as surface 10, is much more pleasingaesthetically when viewed under normal lighting conditions, having avelvet-like appearance rather than the streaky appearance of surface 10.Screen surface 11, like surface 10, comprises a plurality of undulatingcontiguous grooves extending in a vertical direction, as viewed inFIG. 1. However, unlike surface 10, the depth of the grooves comprisingsurface 11 does not periodically vary with a substantially constantwavelength. Rather, the groove depth varies with a random wavelength,within a predefined range of wavelengths, to define opticalmicroelements which vary in size accordingly. Preferably, the range ofmicroelement size is within approximately i 20 percent of a mean size.Moreover, unlike surface 10, the microelement depth (or height,depending on the sense of the microelement, concave or convex) is notuniform over screen surface 11. Rather, the microelement depth varies inproportion to its length, so that each microelement, regardless of size,not only has a contour substantially defined by equations (1) and/or (2)above, but also an optical power like all other microelements. Opticalpower, as used herein, refers to the ability of a microelement toredistribute image flux throughout a predefined solid audience angle.

In accordance with the present invention, projection screens having alight-distributing surface similar in appearance to that illustrated bysurface 11 are fabricated by employing various equipment and techniquesconventionally employed in the sound-recording industry. In FIG. 2, aside elevation of the screemcutting apparatus of the invention is shownin a cutting position relative to a blank master 20 wherein the screenmicroelements are to be formed. While the microelements can be cutdirectly in any readily workable material which itself can be used asthe projection screen, the preferred method of manufacture comprises thefabrication of a screen master in some workable material, such asacetate or wax, from which a negative matrix or press tool of correctcontour can be subsequently made. The negative matrix can then be usedto produce a multitude of positive projection screens by wellknown andeconomical duplicating processes, such as embossing, stamping orinjection molding.

As shown in FIG. 2, the cutting apparatus comprises a conventionalstereo sound recording head which includes a cutting stylus S. While amonaural sound recording head could be used, a stereo head is preferreddue to the high quality of auxiliary equipment available forconventional stereo heads. As in all sound recording heads, the cuttingposition of the stylus is determined by the waveform of an electricalsignal applied to the recording head, such as through input cables 31.The recording head is mounted on a milling machine tool holder 32 by acylindrical fitting 33. Means are provided for controlling the verticalposition of fitting 33 in the tool holder 32 so as to provide a coarse,vertical adjustment of the recording head 30 above the surface of theblank master. The blank master may comprise, for instance, an aluminumplate 36 having an acetate coating 37, the thickness of which issufficient to receive the contours of the projection screen surface.Recording head 30 comprises a cutting assembly 40 having a horizontallyextending support arm 41 which is slidably mounted on precision waysdisposed in a saddle 42. By this arrangement, the horizontal position ofcutting assembly 40 can be varied. Set screws 43a and 43b serve to lockarm 41 in a desired horizontal position. Saddle 42 is pivotally mountedabout pin 44 disposed on recording head 30 so that the cutting stylus S,which forms a part of cutting assembly 40, can be pivoted intoengagement with the master. The rotational movement of a cam 46 servesto raise and lower the stylus relative to the surface of the master bycontacting an arm 47 which is rigidly coupled with saddle 42. Thecutting force applied to the stylus is controlled by screw 48 whichserves to adjust the tension in spring 49. The precise depth of cut iscontrolled by adjustment screw 50 which varies the vertical distance ofthe stylus tip from a small glass ball follower 51 which rides on theuncut surface of the master a short, horizontal distance away from thestylus.

A sound recording head which has been found particularly well adaptedfor cutting projection screen masters is the Westrex Corporation, Model3D StereoDisc. As illustrated in FIG. 3 wherein a simplifiedconstructional diagram of the mechanism which controls stylus movementis shown, each recording channel of the stereo recording head contains amagnetic coil form assembly 60, each of which contains a driving coil 62located in separate pole pieces 64 and 65 which are attached to a singlemagnet 66.

The coil assemblies are attached to the stylus holder through links 68which are stiff longitudinally, but flexible laterally. These links arebraced in the center to prevent excessive lateral compliance. Thisstructure re sults in a stiff, forward driving system with a highcompliance in the lateral direction.

The supporting member for the stylus is shown in FIG. 4. The use of acantilever spring 70 permits the stylus to present a uniform impedanceto complex motions in any direction in the vertical plane. The cuttingtip 72 of stylus S is preferably sapphire, but can be fabricated fromany materail capable of cutting contours in the material used as themaster. The cutting profile of the stylus is designed to conform withthe desired transverse cross section of the screen elements, such asthat defined by equation (1 above. To assist in cutting the workpiecewith the requistie accuracy, the stylus is heated by heating coil 73 toa temperature such as to soften adequately the acetate coating of thescreen blank.

in fabricating projection screen masters by use of the apparatusdescribed above, the workpiece is moved relative to the heated cuttingstylus in a series of equally spaced, parallel traverses. At the sametime, the cutting position of the stylus is electronically variedrelative to the surface of the blank master to produce the desiredlongitudinal cross section or depth profile. Apparatus for moving themaster relative to the stylus is depicted in FIG. 5. During the cuttingoperation, the master is supported by a table which rides atop the crosstravel carriage 81 of an x-y milling table 82. Table 80 is preferablyfabricated from a non-magnetic metal, such as aluminum, so as not tointerfere with the magnetic cutting assembly 40. In the upper surface oftable 80, a circular groove 85 is provided. At the base of groove 85 isan opening (not shown) which communi-- cates with a nozzle 86 located onthe edge of the table. Attached to nozzle 86 via hose 87 is a vacuumsource (not shown). By this arrangement, the master is securely fastenedto the surface of table 80 by a vacuum coupling. Carriage 81 is movablein the x direction and its position controlled with precision by aconventional stepping motor 90 which acts through lead screw 91 (notlabeled). Carriage 81 itself rides atop the longitudinal travel carriage93 of the x-y milling table 82. Carriage 93 is movable in they directionby a hydraulic pneumatic motor 95 which precisely controls the rate atwhich carriage 93 moves via piston rod 96.

To move the cutting stylus in a vertical plane and at a rate which, whenthe screen blank is moved at a constant rate relative thereto, resultsin the longitudinal cross section or depth profile desired, the samesignal must be applied, out ofphase, to both drive coils 62. Moreover,since the stylus is not mounted for vertical movement, but rather forpivotal movement on the cantilever spring 70, so as to traverse anarcuate path, as shown in phantom lines in FIG. 4, it is necessary todrive the stylus with a somewhat different waveform than that whichcorresponds to the longitudinal cross section desired. Referring to FIG.6, when a waveform 97 is applied to the cutting stylus, the resultinggroove has a depth profile as shown in the asymmetrical waveform 98. Tocompensate for the asymmetry, it is necessary to drive the cuttingstylus with a counterbalancing asymmetrical waveform 99 which thearcuate stylus movement converts to the depth profile desired (e.g.,waveform 97). It is interesting to note that in the sound recording art,such asymmetry is automatically compensated for during playback by thepickup stylus which is also pivotally mounted and, hence, moves along anarcuate path similar to that along which the stylus used to cut theoriginal master recording moved. In achieving a desired profile forprojection screens, however, such asymmetry must be compensated for byappropriate circuitry. 1

In FIG. 7, preferred circuitry for driving the cutting stylus in such amanner as to randomly vary the size of the microelements cut thereby,without disturbing the ability of such elements to distribute imagelight uniformly throughout a common solid audience angle, is illustratedin block diagram form. Speaking in terms of a Fourier analysis, thesignal applied to the driving coils of the cutting stylus to producemicroelements having a depth profile defined by equation (2) above isrich in harmonics. Therefore, if thesignal is to pass with undistortedshape through the audio amplifier of the recording head, its fundamentalfrequency must lie near the lower end of the flat part of the pass-bandof the amplifier, but not so low as to become subject to the phasedistortion associated with the low-frequency cutoff characteristic. Forthis reason, the mean frequency of the wave generator 100 used togenerate the basic sinusoidal waveform from which the stylus drivingsignal is ultimately derived was chosen to be 200 Hz, and it followsthan that the length of the microelements is determined by the rate atwhich the screen blank is moved in y-direction past the cutting stylus.In order to randomly vary the size of each microelement by i percent ofa mean size, the frequency of the signal applied to the cutting stylusmust be randomly varied by i 20 percent about 200 Hz, or from 160 to 240Hz. To produce such random variation, the output of the wave generator100 is frequency-modulated by a noise signal. Such a noise signal isprovided by a conventional noise generator 101, the output of which isfed through a low-pass filter 102 having a cutoff frequency of about 20Hz. Capacitor Cl serves to eliminate any do component in the noisesignal. The amplitude of the noise generator 101 is set so that thefrequency excursions of the cutting signal are contained almost entirelywithin the limits of i 20 percent of the center frequency of 200 Hz.After passing through a buffer amplifier Al the low frequency noise isapplied to the wave generator. Generator 100 is of the type whichprovides an output frequency based upon the amplitude of an inputvoltage.

If the frequency of the cutting signal were altered while the amplitudethereof remained constant, the contour of each microelement would varyalong the groove length, and each microelement would distribute imagelight over solid audience angles determined by its particular contour.In order to maintain the image flux-distributing power of eachmicroelement constant, regardless of its size, it is necessary to changethe amplitude of the cutting signal in concert with its frequency, insuch a manner that the amplitude is always proportional to thewavelength of the cut, or inversely proportional to the cutting signalfrequency. This implies that the cutting-signal amplitude must be underthe control of the same modulating (noise) signal thatfrequency-modulates the output of generator 100. Thus, the output of thenoise generator is also applied to an amplitude compensation circuit105, described in detail hereinbelow.

As indicated above, in order to drive the cutting stylus in such amanner as to cut symmetrical microelements along the groove length, itis necessary to apply an asymmetrically distorted waveform to thedriving coils of the cutting stylus which, due to the arcuate movementof the stylus, is converted into the desired groove-cutting stylusmovement. It has been found that the required asymmetry can besubstantially achieved by adding to the sinusoidal output of generator100 a small amount of its second harmonic which is in phase with thefundamental frequency. A conventional squar ing circuit 110 consistingof an analog multiplier module with the same signal applied to bothinputs, is used to generate the second harmonic waveform (sin 2x) fromthe fundamental. Capacitor C2 serves to eliminate the dc component ofthe output of circuit 110 which is (sin x)? Since the midpoint of theresulting waveform lags the sin 2: waveform by 45, it is necessary tofeed the output of the wave generator through a phase-lagging circuit113 before combining it with the second harmonic signal in summingoperational amplifier A5. The output of the summing amplifier A5 is thenfed through a shaping circuit which converts the asymmetrical output ofthe summing amplifier A5 to the asymmetrical waveform required to drivethe cutting stylus. The output of the shaping circuit 120 is then fed tothe amplitude compensating circuit which acts to vary the amplitude ofthe shaping circuit output to a level inversely proportional to itsinstantaneous frequency. The output of the amplitude compensationcircuit 105 is then applied to both input channels of the stereosound-recording ampliifer.

When the output frequency of generator 100 is constant, the phase lag of45 which is required of circuit 113 can be accomplished by a simple RCcircuit. However, when the output of generator 100 is frequencymodulated, such as by the noise generator 100, the reactance of thecapacitor varies with the instantaneous frequency, according to theformula X l/21rfC. This means that, as the frequency varies, both thephase lag and the amplitude of the output vary. To solve this problem, aconstant reactance phase-lagging circuit was devised, such circuit beingillustrated schematically in FIG. 8.

As mentioned hereinbefore, the output frequency of wave generator 100varies in accordance with the noise voltage v,, from amplifier Al. Asshown in FIG. 8, the output of the wave generator is applied to thephase-lag circuit which comprises resistor R10 and capacitor C2.Operational amplifier A4 is a high-input-impedance unity-gain isolationamplifier which ensures that the action of the phase-lag circuit is notaltered by reason of being loaded by resistor R11 and the inputimpedance at terminal Z of module M-l. Module M-l is an analog divider,which acts as an amplifier having a voltage gain, from Z to X, which canbe varied under the con trol of an electrical signal applied to terminalY.

Connected as shown in FIG. 8, module M-l causes the effective value ofcapacitor C2 to be larger than its actual value. Varying the gain ofmodule M-l results in a corresponding change in the effective value ofthe capacitance of C1. The gain-control signal for module M-l isobtained by adding a constant voltage to the modulating noise voltage inoperational amplifier A3. In the preferred embodiment, resistors R7, R8and R9 are selected to make the output of amplifier A3 approxately (33.9 v,,) volts, when the instantaneous frequency of the wave generatoris 200(1 v,,) Hz.

In accordance with the FIG. 8 circuit, the effective value of thecapacitance is changed in concert with the output frequency of the wavegenerator in such a manner that the effective reactance X of capacitorC2 has a maximum change of only 1.6 percent while v, varies over therange ofi 0.2 volts, and the output frequency of wave generator 100varies between and 240 Hz. An uncompensated capacitor would change itsreactance over a total range of 41.7 percent of its median value for thesame frequency changes. Thus, the effect of the constant reactancecircuit of FIG. 8 is to hold both the phase shift and the amplitude ofthe output of amplifier A4 substantially constant while the wavegenerator frequency varies under the influence of the noise voltage. Tovary the amount of second harmonic added to or subtracted from theoutput of amplifier A4, a po- 'tentiometer P1 is connected as shown.

To produce the desired waveform from the asymmetrically distorted sinewave output of summing amplifier A5, output of the amplifier is fed tothe shaping circuit 111. As shown in H6. 10, this signal is segmented byreason of having to overcome successively the forward voltage dropsacross diodes Dl-DIO. Diodes Dl-DS and D6-D10 serve to segment thepositive and nega tive-going portions of the input signal, respectively.Operational amplifier A6 serves to sum the contributions of thevarioussegments to produce a difference signal Ax having a waveformrepresenting the difference by which the desired waveform [equation (2)plus distortion] differs from the asymmetrically distorted sine wave.The contributions of the individual segments to the output of amplifierA6 are adjusted by varying the values of resistors R-l4R-l8. The outputof amplifier A6 is adjustable in amplitude by potentiometer P2. Bysimply adding the differencesignal Ax, which is of a polarity oppositethat of the unshaped signal due to the polarity reversing affect ofamplifier A6, to the unshaped signal, the desired waveform for drivingthe cutting stylus is achieved. Such addition is performed byoperational amplifier A7. Resistors R19 and R20 and potentiometer P3serve to control the gain provided by the summing amplifier A7. Theoutput of amplifier A7 is then fed to the input to the analog dividermodule M-3 of the amplitude compensation circuit.

in order for the cutting stylus to cut microelements of random size butof similar contour, it is necessary to vary the amplitude of thevariable-frequency output of shaping circuit 120 so that the product ofamplitude and frequency is substantially constant. As shown in FlG. 9,the output of shaping circuit 120 is fed to input terminal Z of ananalog divider module M-3. Output X of module M-3 is proportional toZ/Y. The noise voltage v,, from buffer amplifier A1 is fed tooperational amplifier A2 which adds a constant voltage to it, so thatthe output of amplifier A2 is proportional to (l v,,)

R,- lOKohms R, 47Kohms R l mcgohm R, 68 K ohms R, .6.8 K ohms R 39 Kohms- R, .l5 megohms R 100 K ohms R IOKohms R, 5 6Kohms R 47.5 K ohms R5.6 K ohms R .475 megohms R K ohms R 24.4 K ohms R .lO megohms R 94.8 Kohms R 10 K ohms R, l8Kohms P,l0Kohms R,,-.l0megohms Cl 10 1f. R, .IOmegohms C2 .Ol pf R,, .lOmegohms C3 lOpf R 22 K ohms To initiate thecutting operation, a start button is pressed which pivots the recordinghead about pin 44 into a cutting position, causes pneumatic motor 95 tomove carriage 93 in the y direction and causes the above-describedelectronic circuitry to drive the cutting stylus according to thewaveform of the electrical signal applied thereto. After cutting agroove of predetermined length, a microswitch (not shown) is actuated bycarriage 93 which serves to stop pneumatic motor 95, activate a solenoidwhich moves cam 46 of the recording head clockwise into a position topivot the cutting assembly into an inoperative position, and actuatestepping motor so as to move the carriage 81 a predetermined distance inthe x direction. The microswitch also returns the carriage 93 to itsstarting position on the y-axis, which, in turn, actuates a secondmicroswitch (not shown). When actuated, the second microswitch rotatescam 46 counterclockwise to permit the recording head to pivot into anoperable cutting position, and the cutting process is repeated. Thisprocess continues without interruption until the entire screen masterhas been cut.

As the heated stylus S cuts a groove in the screen blank, a continuoussliver or chip is extricated from the screen blank surface. Tocontinuously draw this sliver away from the screen blank, a vacuumnozzle 162 (shown in FIG. 2) connected to a vacuum source through hose163, is positioned adjacent stylus S during the cutting operation.

After making the projection screen master in accordance with theafore-described method and apparatus, projection screens can be producedtherefrom by making a negative matrix or master from the original, andcasting positive screens, in a resinous material,. from the negativematrix. Preferably, the negative matrix is made from General ElectricRTV-60 silicone rubber which is prepared by adding 3 grams of dibutyltin dilaurate RTV curing catalyst to 2 pounds of the RTV-60 rubber,agitating the mixture with an electric stirrer for 5 minutes and placingit in a bell jar which is then evacuated to a pressure of microns ofmercury for about 20 minutes. Upon fixing sidewalls to the edge of theoriginal master, the RTV rubber mixture is poured into this mold. Aftercuring, the rubber mold can then be used to cast positive projectionscreens.

The apparatus and methods set forth above are designed to produce planarprojection screen surfaces comprising contiguous optical microelementswhich, while being of a random size within a predefined size range(determined by the amplitude of the noise voltage), are of substantiallyidentical contour. Moreover, the orientation of each of themicroelements is such that its optical axis is substantially parallel tothe axes of all other microelements. By controlling the angle at whichincoming image-light impinges on each microelement, such as by curvingthe redistributive surface (in the case of a front or reflection-typeprojection screen) or by using the surface in conjunction with aFresnellike lens (in the case of a rear or refraction-type projectionscreen) screen efficiency can be substantially enhanced. Since the solidangle through which each microelement redistributes image-light is afunction of its angle of incidence on the micro-element surface, properselection on the angle of incidence can be used as a means for directingthe redistributed image-light from each microelement toward the audiencespace.

To fabricate such curved projection screens from masters having planarsurfaces, the aforementioned rubber negative mold can be placed on thesurface of a spherical or cylindrical section of desired radius ofcurvature prior to casting. A Maraglas epoxy resin, after beingdegassed, can then be poured into the mold.

After heating in an oven at 200 F for several hours to harden the resin,the casting can be coated with an aluminum coating to form a frontprojection screen.

This invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifica tions can be effected within the spirit and scope of theinvention.

We claim:

1. A method for fabricating a projection screen master having a surfaCecomprising means defining a plurality of parallel contiguous rectilineargrooves, each having a depth which undulates along the groove length, ata randomly changing spatial frequency within a predefined spatialfrequency range, to define a row of contigous optical microelements ofrandom sizes and of substantially uniform optical power, said methodcom- 4 prising the steps of:

cutting rectilinear grooves in the surfaces of a blank projection screenmaster with the stylus of a soundrecording head; and

simultaneously randomly modulating the cutting depth of the stylus in aplane perpendicular to said surface by applying an electrical signal tothe recording head of random frequency and of an amplitude inverselyproportional to the instantaneous frequency of said signal.

2. Apparatus for fabricating projection screens having an imageflux-redistributing surface comprising means defining a plurality ofrectilinear grooves, each groove having a depth which undulates alongthe groove length in such a manner as to define a row of alternatelyconcave and convex optical microelements of random sizes, within apredefined size range, and of substantially uniform optical power, saidapparatus comprising:

groove cutting means-including at least one cutting stylus and controlmeans operatively coupled to said stylus and responsive to an electricalsignal for controlling the cutting position of said stylus; means formoving a blank projection screen master and said stylus relative to eachother to cut a plurality of rectilinear grooves in at least one surfaceof said master; and

circuit means operatively coupled to said control means for generatingan electrical signal for controlling the cutting position of said stylusduring movement of said master surface and said stylus relative to eaChother, said signal having a randomly varying frequency, within apredefined frequency range, and an amplitude inversely proportional tothe instantaneous frequency thereof, whereby each of the grooves cut bysaid stylus has a depth which undulates along the groove length in sucha manner as to define said row of alternately concave and convex opticalmicroelements of random sizes within a predefined size range, and ofsubstantially uniform optical power.

3. Apparatus according claim 2 wherein said moving means comprises meansfor moving said screen master relative to said stylus at a substantiallyconstant velocity.

4. Apparatus according to claim 2 wherein said groove-cutting meanscomprises a sound-recording head.

5. Apparatus according to claim 2 wherein said circuit means comprises avoltage-controlled sine-wave generator, and noise means operativelycoupled to said sine-wave generator for randomly varying the frequencyof the sinusoidal output thereof.

6. Apparatus according to claim 5 wherein said circuit means furthercomprises means for asymmetrically distorting the sinusoidal output ofsaid sine-wave generator.

7. Apparatus according to claim 6 wherein said crircuit means furthercomprises means operatively coupled to said distorting means and saidnoise means for varying the amplitude of the asymmetrically distortedoutput of said distorting means in a manner inversely proportional tothe instantaneous frequency of said distorting means output.

53 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,7 ,17 Dated nuary 29, 197

Inventor(s) Harvey O. Hoadley; Robert N. Wolfe; Beverlv F. Palmer, 7Roger S. Vanheyninga It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1 line 3O, "material" should read -along--;

Column 1, line 36, "requisite" should read --flux--;

Column 6, line 5, "in" should read -In--;

Column 9, line 59, "R 10 K ohms" should read P lO'K ohms--;

Column 11, claim 1', line 11, "surface" should-read "surface";

Column 11, claim 2, line 37, "means-including" should read -meansincluding;

Column 12, line 8, "eaCh' should read --each--;

Column 12', line 18, please insert the word to-- after thewordaccording;

Column l2, line 34, "crir-" should read --cir- Signed and sealed this11th day of June 1971;.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. C. MARSHALL DAHN Attesting Officer Commissioner ofPatents Po-wfio UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent No. 3,7 ,17 I Dated January 29, 197A Inventofls) Harvey 0.Hoadley; Robert N. Wolfe; Beverlv F. Palmer, 1 Roger S. Vanheyningen Itis certified that: error appears in the above-identified patent andthat: said Letters Patent are hereby corrected as shown below:

Column 1 line 3O, "material" should read -along- Column-l, line 36,"requisite" should read --flux--; Column 6 line 5, "in" should read-In--; I

Column 9, line 59, "R 1o roams" should read "P 10 K ohms--;

Column 11, claim 1', line 11, "Surface" should-read surface--;

Column claim line 37, "means-including" should read --means including";i

Column 12, line 8, "eaCh" should read --each--;

Column 12', line 18, please insert the word to-- after the Wordaccording;

Column l2, line 3%, "crir-" should read --cir- Signed and sealed this11th day of June 197k.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. c. MARSHALL 1mm Commissioner of Patents AttestingOfficer

1. A method for fabricating a projection screen master having a surfaCecomprising means defining a plurality of parallel contiguous rectilineargrooves, each having a depth which undulates along the groove length, ata randomly changing spatial frequency within a predefined spatialfrequency range, to define a row of contigous optical microelements ofrandom sizes and of substantially uniform optical power, said methodcomprising the steps of: cutting rectilinear grooves in the surfaces ofa blank projection screen master with the stylus of a sound-recordinghead; and simultaneously randomly modulating the cutting depth of thestylus in a plane perpendicular to said surface by applying anelectrical signal to the recording head of random frequency and of anamplitude inversely proportional to the instantaneous frequency of saidsignal.
 2. Apparatus for fabricating projection screens having an imageflux-redistributing surface comprising means defining a plurality ofrectilinear grooves, each groove having a depth which undulates alongthe groove length in such a manner as to define a row of alternatelyconcave and convex optical microelements of random sizes, within apredefined size range, and of substantially uniform optical power, saidapparatus comprising: groove cutting means-including at least onecutting stylus and control means operatively coupled to said stylus andresponsive to an electrical signal for controlling the cutting positionof said stylus; means for moving a blank projection screen master andsaid stylus relative to each other to cut a plurality of rectilineargrooves in at least one surface of said master; and circuit meansoperatively coupled to said control means for generating an electricalsignal for controlling the cutting position of said stylus duringmovement of said master surface and said stylus relative to eaCh other,said signal having a randomly varying frequency, within a predefinedfrequency range, and an amplitude inversely proportional to theinstantaneous frequency thereof, whereby each of the grooves cut by saidstylus has a depth which undulates along the groove length in such amanner as to define said row of alternately concave and convex opticalmicroelements of random sizes within a predefined size range, and ofsubstantially uniform optical power.
 3. Apparatus according claim 2wherein said moving means comprises means for moving said screen masterrelative to said stylus at a substantially constant velocity. 4.Apparatus according to claim 2 wherein said grOove-cutting meanscomprises a sound-recording head.
 5. Apparatus according to claim 2wherein said circuit means comprises a voltage-controlled sine-wavegenerator, and noise means operatively coupled to said sine-wavegenerator for randomly varying the frequency of the sinusoidal outputthereof.
 6. Apparatus according to claim 5 wherein said circuit meansfurther comprises means for asymmetrically distorting the sinusoidaloutput of said sine-wave generator.
 7. Apparatus according to claim 6wherein said crircuit means further comprises means operatively coupledto said distorting means and said noise means for varying the amplitudeof the asymmetrically distorted output of said distorting means in amanner inversely proportional to the instantaneous frequency of saiddistorting means output.